ES2246708B1 - PROCEDURE FOR OBTAINING SPECTRAL HOLES IN THE TRANSMISSION OF SIGNS BY THE ELECTRICAL NETWORK. - Google Patents

Abstract

Procedure for obtaining spectral gaps in the transmission of signals through the power grid. It allows the transmission of signals through the electrical network with an adequate power spectral density, so that said power spectral density conforms to the regulations of the current regulations by means of the possibility of introducing spectral holes in the transmission efficiently. It is characterized by the adjustment of the power of one or more carriers of an OFDM signal for the generation of said gaps, by the use of IDFTs of at least 1024 points (if the IDFT is complex) or at least 2048 points (if the IDFT it is real), and by the use of a window that multiplies the symbols that will be transmitted in time.

Description

Procedure for obtaining gaps spectral in the transmission of signals through the power grid.

Object of the invention

The present invention, as expressed in the statement of this specification refers to a procedure for obtaining spectral gaps in the Signal transmission over the electricity network. A spectral hole is defined as a frequency band where the power density transmitted is less than a certain required value. This procedure It is especially important in the creation and use of a system of transmission over the electricity grid, since it allows adapting the Signal strength transmitted at different frequencies so that adapts to the regulations that the regulations in force in the different countries set for signal transmission by the power grid, with the appropriate spectral gaps that you specify such regulations. This will prevent interference with licensed services that use the same frequency band that the signal transmitted by the communications system by the power grid

Background of the invention

The means of communication constituted by the network Electrical, both low and medium voltage, is a medium that initially it was not intended for the transmission of signals from telecommunication. This medium is an unshielded medium in which the transmitted signal can radiate and interfere with other systems that are using the same frequencies. It is of relevance the case of radio amateurs and other services of radiocommunications including radionavigation, services emergency, etc. In these services a low interfering signal can interrupt communication, which is especially important to avoid power transmission in said frequencies

That is why the regulations in force in multiple countries impose restrictions on the spectral density of power (PSD) that can be transmitted on certain frequencies, with what is necessary to obtain deep spectral gaps for perform signal transmission over the power grid according With the regulations. The process object of the invention is designed for the creation of deep spectral gaps as efficiently as possible, covering bandwidth strictly necesary.

This procedure seeks the adequacy of the signal transmitted to certain imposed power patterns for the legal reasons mentioned above in an efficient way. These power patterns are usually defined as ranges of frequency at which power transmission is very limited (for example, 30 dBs less than other frequencies) or even forbidden However, given the technological limitations it is not possible to get an immediate transition between a band of frequencies with a certain power spectral density and another with a different power spectral density, which is not possible to abruptly eliminate the signal transmitted in a certain way frequency range without reducing carrier power adjacent to the spectral hole. Thanks to the procedure of invention this reduction can be carried out efficiently, minimizing the number of carriers adjacent to the hole affected by the generation of said spectral hole.

On the other hand, the concepts of poisoning and specifically the raised cosine window, they are not inventions of this procedure, but they are also known in the state of Art. For example they can be found in texts of telecommunication as "Digital Communications" by John G. Proakis of the McGraw-Hill publishing house, in the "Discrete-Time Signal Processing" by Alan V. Oppenheim from Prentice Hall, or "OFDM for wireless multimedia communications "by Richard Van Nee and Ramjee Prasad of the publishing house "Artech House Publishers". In the first reference comments that raised cosine windows are used for the design of limited band signals (to avoid interference between symbols), while in the last reference it is commented that the poisoning process applied to the symbols OFDM individual is applied in reducing the spectrum of the out-of-band signal

The novelty of the process of the invention is to produce spectral gaps (notches) of more than 30 dBs within a band efficiently, by adjusting the power sent in certain carriers, the poisoned symbols OFDM plus cyclic prefix and the use of appropriate values of number of active carriers and IDFT size; thing you can't deduced from the state of the art prior to this patent.

Description of the invention

To achieve the objectives and avoid drawbacks indicated in previous sections, the invention it consists of a procedure to obtain gaps spectral in the transmission of signals through the power grid, which includes sending signals by orthogonal multiplexing by frequency division (OFDM), and where the communication nodes they have a transmitter, means to add a cyclic prefix and Frequency to time conversion means of OFDM symbols.

This procedure is characterized because adjust the power of one or more carriers of the OFDM signal selectively by dimming or by elimination of these carriers, because the transform is applied Inverse discrete Fourier (IDFT) used to transform the signal from the frequency domain to the time domain with a number of points that selectively must be greater than or equal to 1024 if the IDFT is complex or greater than or equal to 2048 if the IDFT is real, and because, once the cyclic prefix is added, they multiply OFDM symbols in the time domain by a window. Thank you to this, it is possible to get abrupt spectral gaps of more than 30 dBs deep, which is a great advantage since enables the transmitted signal not to interfere with others licensed services that use the same frequency bands than the signal that is going to be transmitted by the power grid. Of the Similarly, the process of the invention allows adjusting the power spectral density chosen for the transmission of the signal, thanks to the possibility of introducing spectral gaps within the band

In an embodiment of the invention, the window used is a raised cosine window. This window has been selected for having a flat area in time and due to its spectral characteristics

In another embodiment of the invention, said raised cosine window, which can be defined from its fall factor (rolloff), has a lower or equal fall factor that 0.2.

In addition, the window has a size in number of samples greater than or equal to 2048, where said samples are samples digital pre-analog digital converter (DAC) of the transmitter.

On the other hand, the number of carriers whose power adjusts depends on the situation, width and depth of the spectral gaps and the shape of the window chosen for the process. To produce a depth spectral gap adequate adjusts the power of the carriers located in the spectral hole and a number of carriers adjacent to the previous. The faster the secondary lobes fall of the frequency representation of the selected window, smaller number of adjacent carriers must be adjusted to reach the width and depth of the hole required. To perform the Power setting of the appropriate carriers can be Use several processes that can be combined. The setting can be performed by gradual attenuation, so that the lobe main of the signal transmitted on the carriers within the spectral hollow, or the secondary lobes of the signals transmitted if said carriers are adjacent, are less than the power spectral density (PSD) required for the signal transmitted.

Another way to make the adjustment is the direct elimination of the affected carriers, referring to said  elimination to not transmit any power in the carriers that they will be adjusted, so that carriers are eliminated located within the spectral hole and a number of carriers adjacent suitable to get the spectral density of power (PSD) is less than that required for the transmitted signal in the spectral hole.

Both adjustment processes can be used independently in the necessary carriers or be combined, being able to eliminate certain carriers and attenuate others.

Then, to facilitate a better understanding of this descriptive report and being an integral part of the same, some figures are accompanied in which with character illustrative and not limiting the object of the invention.

Brief description of the figures

Figure 1.- Represents an example of density power spectral (PSD) that the regulations require in the Signal transmission in the 2 to 30 MHz band to protect licensed services on these frequencies.

Figure 2.- Represents an example of a window that It can be used in the process of the invention.

Figure 3.- Represents a block diagram of the  Poisoning process of a sampled signal.

Figure 4.- Represents several typical windows both in time and in frequency.

Figure 5.- Represents windows in raised cosine with several rolloff factors (?) both in time and in frequency.

Figure 6.- Represents a block diagram of the  process of creating spectral gaps in transmission.

Figure 7.- Represents the windows used in the exemplary embodiment to achieve a spectral gap of 30 dBs deep

Description of an embodiment of the invention

Following is a description of a example of the invention, referring to numbering Adopted in the figures.

In signal transmission and the design of communications systems through the electricity network, one of the most important problems is that the transmission medium is not shielded, and part of the signal transmitted to the mains will radiate outward, being able to interfere with systems graduates who use a frequency range overlapping the range of frequencies used in the transmitted signal.

In order to reduce these interferences,  Many countries have regulations on the spectral density of maximum power that can be radiated or introduced into the line electric Unfortunately, these regulations vary from country to country, and therefore it is necessary to create abrupt gaps in the spectrum that can be adjusted to avoid radiate in certain frequencies depending on the country. In figure 1 it can be seen an example of power spectral density that would be required to be able to transmit signals between 2 and 30 MHz. As you can see, there are multiple spectral gaps that must be respected to be able to transmit in said band.

The process of the invention allows to create these spectral gaps in the signal transmitted in a way efficient and thereby adjust to the proper spectral density in any case.

As explained above, the procedure of the invention uses the concept of poisoning, which is known in the state of the art. Unfortunately depending of the technical source chosen this concept can be interpreted in one way or another, so before introducing a exemplary embodiment of the process of the invention, will state the criterion of definition of "window" used in this procedure.

Wrapping a digital signal consists of Multiply the samples of this signal by a window. This window is another digital signal that, in general, has a set of adjacent samples other than zero. In figure 2 you can see an example of a window centered on sample 0.

Once you have chosen the window to use in the process, it is necessary to perform the poisoning. In figure 3 you can Observe the signal poisoning process. This poisoned it is done by multiplying (1) the samples of the signal by the window samples (for which the window is you have to move to the right position, instead of the sample 0). The signal poisoning process (multiplication in the temporal domain) affects the signal in the frequency domain. This  dependence can be observed in the modulation theorem or the poisoned:

and [n] = x [n] v [n] \ leftrightarrow Y (e ^ {j \ omega}) = \ frac {1} {2 \ pi} \ int ^ {\ pi} _ {- \ pi} X (e ^ {j \ theta}) V (e ^ {j (\ omega - \ theta)}) d \ theta

This theorem shows that a multiplication in the temporal domain produces a periodic convolution by the form of the window in the frequency domain. Lobes window childs in the frequency domain determine the frequency form of the output signal and [n]. Figure 4 shows the form in the domain of the time and frequency of some typical windows (triangular, Bartlett and Chebyshev) known in the state of the art. In this figure it can be seen that the shape of the window Determine the secondary lobes of the frequency response. For example, Chebyshev's frequency response remains constant (in the secondary lobes), while for Bartlett it is superior in the initial secondary lobes but falls very Quickly in the following.

For the embodiment described, The preferred window is a raised cosine window. TO then the formula of the raised cosine window is stated centered on the zero sample:

one

Where N is the size of the window defined as  window values with value (v [n]) above 0.5, and? Is the drop factor. In general a window of raised cosine can be defined by said drop factor? (rolloff factor). This factor is always between 0 and 1 and indicates the excess signal that is transmitted. The higher the factor of rolloff, the softer the fall, as can be seen in the Figure 5. This figure shows several cosine windows raised both in time and frequency, and its dependence on the factor of rolloff (β). When the rolloff factor reaches 0, the window becomes a rectangular window that presents secondary lobes very high in frequency.

If the rolloff value is increased, they are achieved less elevated secondary lobes in exchange for sacrificing efficiency. This loss of efficiency is due to the window being Increase in size (where the excess is calculated as N *?). On the other hand, if a rolloff factor that is too low is used, loses efficiency because the secondary lobes do not lower what sufficient and in that case it becomes necessary to increase the number of carriers adjacent to the spectral recess that must be adjusted in power.

A way to increase efficiency maintaining the rolloff factor is to increase the number of OFDM signal carriers. The number of adjacent carriers that  must be adjusted to create a spectral gap will not vary with respect to the previous case, but since the total number of carriers is greater, The loss of efficiency is lower in percentage. That is why the procedure is applicable from a certain number of IDFT carriers.

When making a design for the transmission of signals over the power grid, it has been calculated that the rolloff factor has to be less than 0.2, to have a high efficiency. Similarly, the number of samples of the window should be greater than 2048. For this specific example of embodiment of the invention a rolloff factor of 0.15, a factor of 0.05 and 2600 samples.

To carry out the process of the invention on the signal in transmission (once the window is chosen) a block diagram as shown in Figure 6. In this figure the signal to be transmitted in each carrier of the OFDM signal in a block (2) that adjusts the power of the Suitable carriers to create the necessary spectral gaps. Mainly the carriers located in the hole will be adjusted spectral as well as its adjacent carriers. The higher the fall of the secondary lobes of the raised cosine window chosen, fewer adjacent carriers should be affected to get the spectral hole. Then the block (3) of this embodiment realizes the transformation Fast Fourier Reverse (IFFT) in a complex way that is used conventionally to implement the discrete inverse transform Fourier (IDFT) with OFDM modulations. Then the block (4) Insert the cyclic prefix generated by block (5), and block (6) multiply the digital signal in the time domain by the window generated by the block (7). For the process of generation of spectral gaps be efficient is necessary use an OFDM with multiple carriers and IFFT with a large number of points In this exemplary embodiment, a Complex IFFT 1024 points.

Finally, before transmitting and although this block does not appear in figure 6, the conversion of the digital signal to analog signal through a DAC converter.

To get abrupt spectral gaps is not  sufficient attenuation or elimination of the carriers located in the frequency positions within the spectral gap. This is because there is remaining power due to the lobes secondary of the adjacent carriers that occurs, in their mostly due to transitions of the OFDM signal that reach the spectral area where the hole should be. To avoid it is it is necessary to adjust the power or also eliminate a certain number of carriers adjacent to the carriers located in the hollow end positions. This is what the block takes care of (2) of Figure 6. Depending on the shape of the cosine window elevation generated by block (7), the number of carriers adjacent whose power must be modified is greater or less, so that both blocks are interrelated.

Figure 7 shows the shape of two raised cosine windows with the values chosen in this example of performance (rolloff factor 0.15 and rolloff factor 0.05, and number of samples 2600). In this case, for example, if you want to achieve a spectral gap of 30 dBs depth, we would have to adjust the power of 4 adjacent carriers in the case of use the method of the invention with a cosine window raised with a rolloff factor of 0.15 and a total of 7 carriers adjacent in the case of using a rolloff factor of 0.05.

Claims (8)

1. Procedure for obtaining spectral gaps in the transmission of signals through the power grid, which includes the sending of signals by orthogonal frequency division multiplexing (OFDM) where the communication nodes have a transmitter, means to add a cyclic prefix and frequency-to-time conversion means of OFDM symbols; characterized in that it comprises:

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adjust the power of one or more carriers of the OFDM signal through a process selected from attenuation of carriers and elimination of said carriers;

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apply the discrete inverse transform Fourier (IDFT) to transform the domain signal of the frequency to time domain with a number of points of at at least 1024 if said IDFT is complex, and at least 2048 points if the IDFT is real;

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Add a cyclic prefix, and then multiply the OFDM symbols in the time domain by a window;

to get an effect selected from getting abrupt spectral gaps of more than 30 dBs deep and without interfering with licensed services that use the same frequency bands, achieve compliance with the power spectral density required for transmission of the signal and a combination of same. 2. Procedure for obtaining spectral gaps in the transmission of signals by the power grid, according to claim 1, characterized in that the window used to achieve the abrupt spectral gaps is a raised cosine window. 3. Procedure for obtaining spectral gaps in the transmission of signals by the electricity network, according to claim 2, characterized in that the raised cosine window has a drop factor (rolloff) of at most 0.2. 4. Procedure for obtaining spectral gaps in the transmission of signals by the electricity network, according to claim 1, characterized in that the window has a number of samples of at least 2048, wherein said samples are digital samples prior to the analog digital converter (DAC ) of the transmitter. 5. Procedure for obtaining spectral gaps in the transmission of signals by the power grid, according to claim 1, characterized in that the number of carriers whose power is adjusted depends on the position, width and depth of the spectral gaps, and the shape of the window, for which to produce a spectral hole, the power of the carriers located in that hole is adjusted and the power of a number of carriers adjacent to the previous ones is adjusted, which will be lower the faster the secondary lobes of the window decree used in the transmission process. 6. Procedure for obtaining spectral gaps in the transmission of signals by the electricity network, according to claim 1, characterized in that the adjustment of the power of the carriers is carried out by means of a gradual attenuation, so that the lobe selected between the lobe main of the signal transmitted in the carriers within the spectral gap and the secondary lobes of the transmitted signals if said carriers are adjacent, are less than the power spectral density (PSD) required for the signal transmitted in the spectral gap. 7. Procedure for obtaining spectral gaps in the transmission of signals by the electricity network, according to claim 1, characterized in that the adjustment of the power of the carriers is carried out by means of the elimination of carriers, this elimination referring to not transmitting any power in the carrier to be adjusted, to eliminate the carriers located within the spectral gap and a number of adjacent carriers sufficient to achieve that the power spectral density (PSD) is less than that required for the signal transmitted in the spectral gap. 8. Procedure for obtaining spectral gaps in the transmission of signals by the power grid, according to claims 6 and 7, characterized in that the adjustment of the carriers is carried out by a process selected from gradual attenuation, elimination, and a combination of both.